option strict on disallow late binding

[FlyingBear]

I have a webbrowser in the form, ele is a HtmlElement on the page in the browser. now, I want to get ele's value (if it exists). if I turn off option strict, then
If (ele.DomElement.value() IsNot Nothing) Then
val = ele.DomElement.value.ToString.ToLower()
End If

this works. but if I turn on option strict, then it has error for "disallow late binding".

I guess I should use ctype to convert ele.domElement, but what type should I convert it to? htmlelement will not work.


thanks

[jmcilhinney]

You have to reference the COM library MSHTML.dll to be able to refer to the actual type that DomElement would return, as the documentation for the HtmlElement.DomElement property clearly states. I'll say it again: always read the documentation first.

[FlyingBear]

I actually checked online before asking question. the problem I have is: once I cast ele.DomElement to IHTMLElement (or IHTMLElement2, 3...), I can not find that they have the property for .value (corresponding to ele.DomElement.value()).

Maybe need cast it to other class/interface?


thanks

[jmcilhinney]

Add the reference but don't change the code, then put a breakpoint on that line and run the project. When the debugger breaks, what does it say the type of ele.DomElement is?

[jmcilhinney]

I just created a new project and added a reference to MSHTML.dll. I then opened the Object Browser and expanded the relevant nodes. Presumably, if you're getting the 'value' property, this object represents an 'input' element in the HTML code. I looked down the list of types and paid attention to those that appeared relevant. I found the IHTMLInputElement interface, which has a 'value' property. I'm guessing that that's the type you want.

[FlyingBear]

thanks. that's a good idea.

it turns out that I may have to use late binding. for example, I need search an object with name "f", I do not know the object is an IHTMLInputElement or IHTMLButtonElement, or else. So, I can not simply cast it to one of them.

thanks, really appreciate the help

[jmcilhinney]

If you know that it's got a 'value' property then you presumably know that it must be one of a subset of types that has a 'value' property. In that case you do something like this:

Dim input = TryCast(element, IHTMLInputElement)
 
If input IsNot Nothing Then
    'Use input here.
 
    Return
End If
 
Dim button = TryCast(element, IHTMLButtonElement)
 
If button IsNot Nothing Then
    'Use button here.
 
    Return
End If
 
'Etc.

If you think that that's too laborious then you could certainly stick with late-binding. If you do though, make sure that you set Option Strict On for the project and then create one or more separate code files, declaring a partial class if appropriate, to contain just the code that specifically requires late-binding. You can then set Option Strict Off for just those files.

[FlyingBear]

thanks. I will go for late binding because there are too many places to do this cast (the code was developed by others and has many lines, )

thanks again

[jmcilhinney]

The fact that it's in a lot of places doesn't necessarily mean it's a lot of work either way. All you need is one function. If you want to use early-binding all the way:

Friend Function GetElementValue(ByVal element As HtmlElement) As String
    Dim domElement As Object = element.DomElement
    Dim input = TryCast(domElement, IHTMLInputElement)
 
    If input IsNot Nothing Then
        Return input.vlaue
    End If
 
    Dim button = TryCast(domElement, IHTMLButtonElement)
 
    If button IsNot Nothing Then
        Return button.value
    End If
 
    '...
 
    Return Nothing
End Function

Wherever you want the value from an HtmlElement, you simply call that method and pass the HtmlElement as an argument. If you want to use late-binding then, as I said, create a separate code file and set Option Strict Off at the top, then add this function:

Friend Function GetElementValue(ByVal element As HtmlElement) As String
    Return element.DomElement.value
End Function

Now you can leave Option Strict On for the project and you retain its benefits for the rest of your code. Again, simply call that function anywhere you need the value from an HtmlElement. It should probably be a simple Find & Replace operation to add the method calls.

[Joacim Andersson]

If you want to use late binding put the code that uses it in it's on Partial Class file and only turn Option Strict off in that file. Shouldn't be more than one method, right?